ThirdLove— a women-led startup known for introducing half-cup sizes into the mainstream — has been one of the Insider Picks teams' favorite lingerie companies to recommend since its inception five years ago. In an extensive testing process, we ranked it #1 in both our definitive buying guide to the best bras you can buy and the best strapless bra you can buy.

The reason why is simple: ThirdLove bras are a rare intersection of affordability, comfort, style, and an intuitive design that makes for a huge improvement on the standard options. Shopping here also includes the often invaluable perk of an energetic customer service team and a 100% fit guarantee. (If you take the company's Fit Finder and order the wrong size, ThirdLove will work to get you into the perfect fit at no further cost, donating the bras you didn't keep to a women's shelter if necessary).

Plus, the company recently expanded to carry 70 sizes, well beyond the industry average of 30 — a launch that racked up a waiting list of 1.3 million women.

Now, at the repeated request of customers, ThirdLove has launched its first-ever Cotton Collection— made from super-soft, breathable Pima cotton that's perfect for all-day wear. Pima cotton, used to create underwear by other cult-favorite startups like Everlane, is a smoother, softer, and more durable variety of cotton thanks to 50% longer fibers. It retains its shape, doesn't pill, and feels light against the skin.

As an online company, ThirdLove relies upon millions of data points from customer feedback to engineer its new products — and for the last four years, one request has been constant: a cotton bra that women can live in. For the last two years, ThirdLove has been designing its response.

The finished product has ultra-thin memory foam cups, two-way convertible straps (worn either straight or crisscrossed), gold alloy strap adjusters, and a keyhole detail. The band has a subtle sheen, and the foam-padded hook-and-eye closure is tagless for a comfortable, scratch-free wear all day.

To give you an idea of what the 24/7 Cotton T-Shirt Bra is like in real life, five women on the Insider Picks team tested it out in real life. Below, you can find our thoughts on how it stacked up. If you'd rather skip to shopping, the short version is this: It's breathable, soft to the touch, and you might forget it's on. The design is basic, but that's fine with us for a T-Shirt Bra this comfortable.

The Cotton Collection is available in four colors: Sea Salt, Ballet, Olive, and Black. Bras are available in sizes A-G (DDDD), and underwear is available in sizes XS - 1X (00 - 16).

I've loved nearly every ThirdLove bra I've tried, and the 24/7 Cotton T-Shirt Bra doesn't disappoint. It's so comfortable that I forgot I was wearing it, which is something I've only ever been able to say about ThirdLove's bras. The fit is absolutely perfect, so I don't need to constantly adjust falling straps or chafing underwire throughout the day. The new cotton fabric feels soft and natural against my skin and contrasts nicely with the more common lacey synthetic fabrics most bra makers use. True to its name, you can't see this bra under a T-shirt, thanks to its smooth, flat seams.

Mara Leighton, Insider Picks reporter:

ThirdLove's 24/7 Cotton T-Shirt Bra is soft to the touch, supportive, and comfortable for all-day wear. It's also true to size. The main standouts for me were that, at 32E, the 24/7 bra's support system was successful without feeling restrictive or creating unflattering rolls under a T-shirt — both of which are my standard complaints. I was also impressed by the design — typically, when the bridge between the two cups isn't separate, it refuses to lay flat against my skin despite the cup sizes being correct. However, single-piece constructions also make for a smoother, better shape under T-shirts, and ThirdLove's version manages to do both — a smooth silhouette and a perfect fit.

It's also a bra you may forget you're wearing. The ultra-thin memory foam cups form nicely to the body, and the material is breathable enough to auto-regulate for the most part. In terms of looks, the keyhole detail in front also adds a fun point of interest to an otherwise basic and functional bra.

I think the sign of a good bra is that it's comfortable enough that you barely realize it's there and you're not itching to take it off the moment you get home. The 24/7 Cotton T-Shirt Bra from ThirdLove delivers on that, with a simple style and soft feel that you'll enjoy wearing every day. The thin cups are comfortable and also form-fitting, so you don't have to worry about any of those annoying outlines under your shirt. I sometimes opt for a larger band size, but I decided to stick to a 32, although I was a little skeptical. I was pleasantly surprised at how comfortable the band was and how smoothly it fit. I actually like the really simple design and I think the keyhole detail is a cute addition if you're looking for a little something extra. This has definitely earned a spot in my everyday rotation.

The 24/7 name is no exaggeration: You really will want to wear this great everyday bra all the time because of how soft and comfortable it is. The memory foam cups fit closely to your chest and they're cut to an attractive shape that should work under most types of fabrics and clothing styles. Personally, I could do without the extra band because it tended to fold over onto itself whenever I wasn't standing or sitting completely straight; however, I do have a smaller chest and I assume it's there for support reasons. Either way, I still love this reliably comfortable bra.

Grace High, Insider Picks analyst:

This is a super comfortable bra — and perfect for everyday wear. For the price, I wish it were a bit more attractive (for example, a decal or something in the center by the cutout). The cups run a bit large, so if you're in between cup sizes, I would recommend sizing down. However, the band size is perfect and there's a comfortable amount of stretch.

Subscribe to our newsletter. Find all the best offers at our Coupons page. Disclosure: This post is brought to you by the Insider Picks team. We highlight products and services you might find interesting. If you buy them, we get a small share of the revenue from the sale from our commerce partners. We frequently receive products free of charge from manufacturers to test. This does not drive our decision as to whether or not a product is featured or recommended. We operate independently from our advertising sales team. We welcome your feedback. Email us at insiderpicks@businessinsider.com.

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